A Measure of Free Trade – the Origins of the Metric System

A Technical Briefing by Robert Yeo, Pro-Lite Technology Ltd August 2019

Defining the “Metre” Introduction For the new system to work, there had to be a A 250 gram piece of Roquefort purchased in France realisable base measurement. In other words, the should weigh exactly the same as a 250 gram piece metre needed a definition. The Academy of bought in New York City, although the American Sciences recommended that the definition be version will be labelled as weighing 8 ounces. The equal to one ten millionth of the distance on a point of this (somewhat cheesy) reference is that meridian between the north pole and the equator, sharing specifications in an unambiguous way is a and this was accepted by the French National basic tenet of modern business, but this was not Assembly in 1791. To determine the length of the always so. For the origins of modern commerce we metre, a survey was commissioned in 1792 from must look to Napoleon Bonaparte, whose actions – the French astronomers Delambre and Mechain in part – laid the foundations for free trade and the who travelled between Dunkirk in France and modern day European Union. Barcelona in Spain. This work was completed in 1798. In December 1799, Napoleon made the The Origins of the Metric System metric system compulsory in France. So was born the metric system which was created “for all people, for all time”. Egalitarian? For sure. The green shoots of a standardised system of Universal? Not exactly. weights and measures can be found in 1668. In that year, Bishop John Wilkins, founder of the British Royal Society, proposed a framework of Britain Joins the Metric World measurements which historians consider contained the basic principles of the modern day Britain had adopted the standard Imperial unit metric system. Actual adoption of a metric system system in 1845 and the move to a metric system first occurred in France as one of many reforms was agreed as long ago as 1965. The European that came into being around the time of the French Union issued the Units of Measure Directive in Revolution. Before then, a variety of disparate 1979 which sought to achieve a common system of systems of measure had existed, which acted as weights and measures to foster free trade. While barriers to free trade. In 1793, a commission of the the UK completed its legal transition to SI units in French Academy of Sciences declared that the 1995, a few imperial units are still officially standard unit of length would be called the sanctioned. For example, draught beer must be “metre”, a word which derives from the Greek sold in pints, the distances shown on road signs metron, meaning "a measure." must be in yards and miles and road speed limits must be given in miles per hour.

www.pro-lite.co.uk Tech Brief: Light Field Imaging Page 1

What Could Possibly Go Wrong? Imperial and metric units, and in any case the consequences of a mistake are unlikely to be life threatening. However, we do often see The importance of a uniform system of weights and misunderstandings in the parameters used to measures was recognised in the United States as describe the output of light sources. We get asked long ago as 1790. At that time, President George about measuring the “brightness” of an LED, but Washington referred the subject of weights and the trouble is that brightness in this context has no measures to his Secretary of State, Thomas specific meaning. We could take brightness to Jefferson, but for a variety of political as well as mean the total luminous flux of the emitter (which practical reasons, the US has retained the British is measured in lumens), or we could assume that Imperial system to this day. For those of us who we are talking about the luminous intensity routinely work with US suppliers, we become (candelas). Maybe the customer has a totally adept at converting between Imperial and metric different understanding altogether. Surprisingly, or SI measures. However, things can (and this seems to be a widespread problem which sometimes do) go disastrously wrong. probably explains why we have seen so many scientists and engineers enrol on our Practical Light Take the case of the Mars Climate Orbiter. The Measurement training workshop. http://www.pro- spacecraft was lost in 1999 when it crashed onto lite.co.uk/File/training_overview.php. the surface of Mars as a result of confusion between Imperial and metric units. The navigation Pro-Lite's workshop considers the question of why software for the spacecraft was developed at the we see a convoluted correlation used to describe NASA Jet Propulsion Laboratory and assumed that the output of so-called “Energy Efficient” lamps. the thruster impulse data would be given in the The specifications printed on the packaging of a metric Newton-seconds. However the contractor compact fluorescent lamp (CFL) may read who built the Orbiter provided the values in the something like: “this 12W CFL has the same power Imperial units of pounds-force seconds which as a 60W light bulb”. In the first instance, this is caused the impulse to be interpreted as about a factually inaccurate; a 12W CFL has the same quarter of the required value. luminous flux as a 60W incandescent lamp. Second, there is a perfectly good unit called the As a further example of metric confusion, consider lumen that we should use directly. In other words, the Boeing 767 aircraft belonging to Air Canada a 12W CFL produces the same number of lumens which in July 1983 earned the sobriquet “Gimli as a 60W incandescent. So this is why the EU Glider” on account of it completely running out of Energy Efficiency Directive requires that fuel at 41,000 feet (sorry, 12,500 meters). manufacturers specify lamps in terms of the light Fortunately, the crew were able to avert disaster emitted (luminous flux in lumens) rather than the by gliding the aircraft to the former airbase at electrical power dissipated (Watts). Gimli, Manitoba. Canada had recently adopted the metric system and human error lead to a miscalculation of the amount of fuel required. Conclusions

The Language of Light Metrology In conclusion, I quote Lord Kelvin who said, “when you can measure what you are speaking about and express it in numbers, you know something about Which brings me to photometry and the it”. Just make sure you stick to the metric system measurement of light, an area in which my and refer to "lamps" not "bulbs", the latter being business (Pro-Lite) specialises. We don’t often see what you put in the ground. problems resulting from confusion between

www.pro-lite.co.uk Tech Brief: Light Field Imaging Page 2